Screen printing has a variety of uses in printing art designs or information on various materials such as clothing, garments, posters, signs, and objects made of glass and metal. The most important advantage of screen printing is that it provides a high quality print using equipment that is relatively uncomplicated and inexpensive compared to other printing methods. A typical print screen used for screen printing is composed of a frame made of wood or metal with a fabric or metal mesh print screen mounted tautly in the lower portion of the frame. In screen printing, a positive of the artwork to be printed is positioned onto the screen which has been prepared with a light sensitive emulsion. The screen is then processed or exposed to light, hardening the unprotected areas of the screen such that they are no longer porous. The areas protected by the positive image are not hardened and can be rinsed porous thus creating a negative of the desired artwork on the screen. This negative image is then used to print the image on the desired fabric, garment or other imprint material. Usually, this is accomplished by positioning the screen against the imprint material and using a squeegee to force gelatinous ink through the uncovered areas of the screen. When the screen is removed the desired image is left on the imprint material. One skilled in the art can see that there are other ways of producing the positive image onto screens. To obtain a high quality image in screen printing it is desirable for the screen image to register precisely with a predetermined print area on a platen (and thus on any garment or substrate to be imprinted that is placed on the platen). Registration in such a manner permits the printed image to be placed at exactly the same location on successive substrates. Precise registration is particularly important when a series of prints must be made on the same garment or piece of substrate to provide a multi-colored image. In such a case even minute displacement of one colored image can significantly affect the image quality.
The most common type of existing screen printing press is the manual rotary press. A manual rotary press is composed of a series of platens radiating angularly from a vertical rotating shaft. Mounted above each of these platens is a screen frame holding assembly. The screen with the desired image is mounted into a screen frame holding assembly and then rotated so that it is positioned above the printing platen and lowered into place. The disadvantage of manual rotary presses is the large amount of space that is required for the spoke-like construction of the platens and for the mounting and rotation of the frame holding assemblies. Moreover, many manual rotary presses rely on precision in the rotation and clamping systems to maintain registration; such systems are susceptible to wear and even minimal amounts of looseness in the assembly can cause significant problems in image quality. In addition, the use of such a system involves significant on-press setup time to accurately mount and register all of the required screen frames before a single print can be made.
Many existing screen printing machines require registration of screens to be conducted on the printing press itself. In one common method of registration the images are roughly centered on the print screens and then the screen frames are inserted into a screen holder. Each screen in its holder is then positioned on the printing press and a sample print is made. Using trial and error, the screen is repeatedly adjusted in the screen holder, clamped down, and reprinted until the image is in registration. Depending on the experience of the operator, the trial and error procedure of registering the images on the printing press can take a long time and requires someone experienced in the art to line up the images by eye. Setting up the images on the press also dictates that the press be non-operative during that time resulting in increased costs from press downtime. Another variant of on-press registration involves taping the positives onto the printing platen area and then aligning the screen, which contains the negative image, to the positive. While this method does not rely on repeated test printing of the images it is still time consuming and requires great care and technique, as even a slight offset in aligning any of the positives in the print area will affect image quality. In an effort to improve the operator's ability to line up the images by eye, some printing machines use micro-registration devices to facilitate adjustment of the screen image by minute fractions of an inch. Such micro-registration devices still require experience to operate and do not result in significant reduction in the time needed to register images as the process relies on trial and error and minuscule adjustment of knobs and clamps. In addition, existing micro-registration devices usually only permit limited range of movement through discrete adjustments along X and Y axes.
Because of the problems associated with on-press registration, methods have been developed that attempt to pre-register the image off-press. One method involves the use of a carry sheet to maintain alignment of the images during creation of the negatives. A carry sheet is a sheet of vellum or cellophane that has a set of pre-punched holes at the top and which is slipped securely underneath the positive for support. Carry sheets suffer from a host of problems. The positive can move on the carry sheet if the tape or other adhesive method used to adhere the positive to the carry sheet, fails. The carry sheet is cumbersome because it must be significantly larger then the positive and the added material makes butt registration more difficult. Finally, carry sheets represent and extra cost and are not very accurate, especially when the operator wants to re-align the positives for reprinting, as the sheets are susceptible to binding, folding and changing shape. Another method of off-press pre-registration involves separately registering the image within the screen frame and then registering the screen frame to the printing platen area. This two step registration process results in less downtime on the printing press. Existing systems involve the attachment of various brackets to the screen frame which are then mated with corresponding connectors on the print head. Such systems are complicated and increase the time required for printing as the brackets must be precisely attached or registration can be affected. In addition, the use of such brackets is impractical for screens of varying sizes and shapes or on warped or damaged screens.
As stated above, multi-color, screen printing is typically done on screen printing presses having platens or pallets indexing substrates on the pallets through an endless path through a number of printing stations along the endless path. Different colors and/or images are printed at the different stations. In rotary, multi-color, screen printing presses of the automated kind, a stepping mechanism steps the pallets from one printing station for one color to and through another printing station for printing another color. Typically, pivoted printing heads at each printing station are heavy and carry motor-driven squeegee carriages with squeegees therein. These machines became quite heavy, large and expensive. In addition, they require considerable set-up time and cannot be quickly changed from one small job to the next small job. Although they are quite fast when properly set up, the set-up and take-down time make them inefficient for small jobs where set-ups and take-downs are frequent.
Another problem for screen printers who have large rotary screen printers is that of printing samples or proofs, in order to sell a job to a customer or to obtain the customer's approval of the multi-color printing on the substrate, such as T-shirt or the like. Often, only a dozen or so samples are printed. The interruption of the production time of a large screen printing apparatus to set and run a dozen samples as well as the time to break sown the press and to return the screen printing press to production is a large cost to the screen printer. A screen printer may have to make samples, or proofs, for four or five customers and await their commissioning full production.
Another form of the multiple screen printing press is the oval press wherein pallets carrying the substrates travel in a horizontal plane carrying substrates seriatim through printing stations, each having a large motor-driven squeegee assembly. Like rotary presses, the oval rotary presses have large, pivoted or rectilinearly traveling, printing heads at each printing station. These oval machines are large in size, heavy, relatively expensive, and require considerably set-up and take-down time like the rotary, multi-color screen printing apparatus. They are not very efficient in screen printing small jobs and are not very efficient in printing small order samples, or proofs, for customers.
At the other end of the screen printing spectrum are manual, small screen printing presses often used to print a single color or multi-color jobs having a manually activated squeegee and a manually indexable table supporting one or more pallets for indexing into the printing station. Such machines are small, inexpensive and relatively efficient for small jobs; but they lack the speed of operation of the larger, expensive, fully automated, rotary or oval screen printing presses. Moreover, they may not be precise enough in their printing to be used for samples, or proofs, which are intended to be run on large, expensive machines.
Thus, there is a need for an economical, multi-color, screen printing system that is efficient in forming and preregistering screens in cassettes and that is efficient in storing and using these cassettes for printing samples, or proofs, for a number of customers and small jobs for various customers.
In recent times, screen printers have seen profits fall as they are forced to print multi-color jobs, that are burdened with increased costs from additional setup time, are preparation or screen preparation, at a price that was once reserved for single color designs. The demand for multi-colored prints has caused many small screen printers to end up competing against larger screen printing companies that, by nature of their size and resources, have more and better equipment and who deal in volume. The average buyer has come to expect a two and three color job for the same price of a one-color job. To compete small screen printers have been forced to reduce or eliminate charges for services such as screen preparation, artwork, ink changes and multi-color prints. The subject invention was developed to address the problem of escalating cost and production times in the screen printing industry, allowing multi-color jobs to be more competitive, and to improve upon existing pre-registration systems which are complicated, time consuming, inaccurate and labor intensive.